1. Field of the Invention
This invention concerns an intermediate frequency detection circuit of a radio receiver which is ensured to generate an output indicative of the reception for broadcasting waves with no erroneous operations caused by suprious signals or the like.
2. Description of the Prior Art
Electronic tuning type radio receivers capable of automatic search channel selection or the like recognize the tuning to a broadcasting wave when an intermediate frequency stage generates an output. The frequency sweeping is then interrupted and a receiving state is entered. The intermediate frequency stage generates its output by the reception not only of broadcasting waves but also of harmonic waves (sprious signals) thereof to interrupt the channel selection operation. A circuit for taking out the intermediate frequency output for channel selection (hereinafter referred to as IF detection circuit) is, desirably, designed so that it produces no output upon reception of the harmonic waves or noises. Moreover, while the IF detection circuit is adapted to monitor the output from the intermediate frequency stage at a certain threshold level and generates an output when the intermediate frequency output exceeds this level, this system results in a problem that a frequency width exceeding the threshold level changes depending on the level of the input signals. If the frequency width becomes broader exact tuning is of course made difficult. It is, accordingly, desired for the IF detection circuit to output a narrow width pulse at an exact tuning point.